I wasn't completely up to date on the stereotypes, myself, so I wasn't actually wondering that.

Anyway, they don't seem to have a very good reason for either of these countervailing tendencies. But, to me, it looks like you could explain a lot by a different old stereotype - namely that men tend to be more attracted by visual features. This might result in those who are trying to attract men - namely straight women and gay men - working harder to achieve slenderness.